If you're a fitness fiend or working to improve your health, you're probably always looking for ways to run faster and farther, train harder and push your limits. You might even already track a number of fitness markers -- heart rate, calories burned, steps walked. You might even know your basal metabolic rate. But there's another key fitness metric you might not know yet: Your VO2 max. 

VO2 max can give you important insights about your cardiorespiratory fitness, such as how long you can sustain a certain intensity of exercise, which relates to fitness hallmarks like your mile run time. Follow along to learn what exactly VO2 max is, how to measure it and how to improve yours. 

Read more:  Most People Aren't Tracking This Key Health Metric (and They Should Be)

What is VO2 max?

VO2 max refers to the maximum amount of oxygen you can utilize during exercise. It's commonly used to test the aerobic endurance or cardiovascular fitness of athletes before and at the end of a training cycle. VO2 max is measured in milliliters of oxygen consumed in one minute, per kilogram of body weight (mL/kg/min). 

It's not the same thing as heart rate, though it can be just as effective, if not more so, to measure and track your fitness progress. VO2 isn't excess post-exercise oxygen consumption (EPOC), which refers to the increase in oxygen your body uses after a workout, not during. 

But don't confuse VO2 max with the lactate threshold, the point during exercise where lactate builds up in your bloodstream faster than your body can expel it. When you reach your lactate threshold, you get that familiar burning or cramping feeling. You reach your lactate threshold at about 50 to 80% of your VO2 max.

How do you test VO2 max?

Although VO2 max is a good marker of fitness, it does present some downfalls. You can't really get an accurate measure of it except in a lab with expensive clinical equipment which is why VO2 max is usually a fitness marker reserved for elite and professional athletes. 

However, some gyms and holistic health clinics offer VO2 max testing for their members or patients. For instance, TriFitLA, a studio gym in Los Angeles, offers VO2 max testing along with several different performance and health tests. If you're really interested, your best bet is to search "VO2 max testing near me" on Google.

To measure VO2 max, you wear a mask and heart rate monitor hooked up to a treadmill or stationary bike. The mask is connected to a machine that collects and measures the volume of oxygen you inhale, and the amount of air you exhale. You'll slowly increase exercise intensity on the treadmill or bike -- getting faster and/or adding more resistance -- until your oxygen consumption remains steady despite an increase in intensity. 

Once you reach that plateau, your body moves from aerobic metabolism to anaerobic metabolism -- that is, your body stops using oxygen to fuel the breakdown of carbohydrates, amino acids, and fats because there isn't enough oxygen there. 

Shortly after you reach that switch, your potential workload plateaus and muscle fatigue sets in. You have to return to an aerobic state of movement to keep going. 

What should my VO2 max be?

Like heart rate, there's no one "good" VO2 max. Your VO2 max will differ from someone else's based on age, gender, fitness level and outside factors like altitude. For example:

• The average sedentary (inactive) male achieves a VO2 max of about 35 to 40 mL/kg/min, and the average sedentary female scores approximately 27 to 30 mL/kg/min.
• Elite male runners have shown VO2 maxes of up to 85 mL/kg/min, and elite female runners have scored up to 77 mL/kg/min.
• A good VO2 max for a 25-year-old male is 42.5-46.4 mL/kg/min, while a good value for a 25-year-old female is 33.0-36.9 mL/kg/min.

Until recently, the highest VO2 max ever measured was that of Bjørn Dæhlie, an Olympic skier who reportedly achieved a VO2 max of 96 ml/kg/min in the 1990s. A young cyclist from Norway reportedly ousted Dæhlie with a VO2 max of 97.5 ml/kg/min, though scientists are still trying to figure out if that reading was accurate. 

How to increase your VO2 max

If you ever do get around to visiting a sports performance lab and getting your VO2 max tested, it'd be worth it to act on that number. Increasing your body's capacity to utilize oxygen is one surefire way to get you closer to your endurance-related goals -- a higher VO2 max essentially extends your breaking point.

You might not be too surprised to learn that high-intensity interval training is one of the best ways to improve your VO2 max. It works because you train your body to work at incredibly high levels for a period of time just long enough to push or surpass your anaerobic threshold before returning to a steadier, aerobic state. 

In a theoretical sense, any exercise that pushes your limits can increase your VO2 max. Think of it like building muscle: Muscles won't grow unless they're exposed to workloads that challenge them. If you never increase your weight on the barbell, you'll never get stronger. 

The same goes for VO2 max -- it's like a muscle of its own. If you run at the same easy pace for the same amount of time every day, you won't get faster or better at running. 

Instead, try adding intervals to your run. For example: 

• Run fast for one minute
• Jog slowly for two minutes
• Sprint for 30 seconds
• Jog slowly for two minutes
• And so forth

If running isn't your thing, you can apply the same principles to swimming, cycling, rowing, or any cross-training activity. 

Ready to start working on your VO2 max? Check out Peloton, Daily Burn and more — 8 of the best fitness subscription apps available.

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The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.